1. Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to a method and apparatus for minimizing shifts between the focal point and the reference mirror caused by thermal effects in the optical microscope objective of an interferometer.
2. Description of the Related Art
Many interferometric devices utilize microscope objectives for focusing beams of light on a sample surface and a reference surface to produce interference fringes representative of the optical path difference (OPD) between the two. As illustrated in simple schematic form in FIG. 1, typical interferometric apparatus 10 consists of a light source 12 directing a beam L of light through an illuminator 14 toward a beam splitter 16, which reflects the light in the direction of a test surface S. The light reflected by the beam splitter 16 passes through a microscope objective 22 focused on the test surface S and incorporating an interferometer (not seen in the figure). The interferometer, such as a Mirau device, for example, comprises a beam splitter and a reference mirror, such that the light beam directed to the sample surface S is split and also directed to the reference mirror. As is well understood by those skilled in the art, the light beams reflected from the reference mirror and the test surface S (the reference and test beams, respectively) are combined to produce interference fringes as a result of the optical path difference between the reference mirror and the test surface S. The light is typically passed back through the interferometric microscope objective 22 and through the beam splitter 16 toward an imaging array 24 positioned in a camera 26 in coaxial alignment with the objective 22. The imaging array 24 typically consists of individual charge-coupled-device (CCD) cells or other sensing apparatus adapted to record a two-dimensional array of signals corresponding to interference effects produced by the interferometer as a result of light reflected at individual x-y coordinates or pixels in the surface S and received at corresponding individual cells in the array. Appropriate electronic hardware (not shown) is also provided to process the signals generated by each cell and transmit them to a computer for further processing. Thus, an interference-fringe map is generated by detecting the intensity of the light signal received in each cell of the array 24. Additional information about the test surface S can be gained by varying the OPD between the reference and test surfaces with a scanning device (not shown) translating either the reference surface or the test surface.
The present invention is directed at an unsolved problem caused by thermal effects in the typical interferometric objective 22 of the interferometer. As illustrated schematically in FIG. 2, a conventional interferometric objective 22 consists of microscope-objective optics 30 adapted to focus the light beam L at a test focal point P on the test surface S of the object being measured. For the purpose of this disclosure, the light beam L is assumed to come from an object location an infinite distance behind the microscope objective, thereby locating the test focal point P defined here at the focal point of the objective. As one skilled in the art would readily understand, though, the disclosure applies equally to the case where the source of beam L is located a finite distance behind the objective and the test focal point P does not coincide with the focal point of the objective. Thus, while the "infinite conjugates" imaging condition, as understood in the art, is used here to simplify the description of the invention, the description can be generalized with no loss of accuracy to the "finite conjugates" imaging condition.
The optics in the objective 30 are rigidly housed in a coaxial support sleeve 32 substantially along the entire length of the objective. A focusing sleeve 34 is mounted on the support sleeve 32 so that it can move along the optical axis X of the assembly for focusing the light beam L at the test focal point P. To produce the fine adjustments required for interferometric applications, the focusing sleeve 34 is typically coupled to the support sleeve 32 by means of a threaded engagement (seen in FIG. 12) along the interior of the sleeve 34, such as on the inside surface of the end collar 36.
As also shown in the enlarged partial view contained in FIG. 2, the focussing sleeve 34 includes a beam splitter 38 and a reference flat 40 with a reference mirror 42 attached to the reference surface 44. By rotating the focusing sleeve 34 with respect to the support sleeve 32, their relative axial position is shifted as needed to focus the light beam L on the reference surface 44. Obviously, the threaded engagement between the two structures is provided with the pitch necessary to produce the focusing adjustments required by the application. By design, once the beam L is focused on the object to be measured, the light reflected from the reference mirror 42 and from the test surface S is combined at the surface 46 of the beam splitter 38 and reflected back through the objective 30 for the necessary interferometric measurements. Therefore, it is critical for good interferometric measurements that the distance between the surface 46 on the beam splitter and the reference mirror 42 remain the same as the distance between the surface 46 and the test focal point P (that is, the reference focal point P' must remain on the mirror 42). For the purposes of this disclosure, the distance between the splitter's surface 46 and the reference mirror 42 is defined as the "reference length" and the distance between the surface 46 and the test focal point P is defined as the "test length." Thus, the reference length is selected by design such that the position of the reference mirror 42 is conjugate to the location of the test focal point P after the focusing of the objective on the test sample.
In practice, thermal effects cause component shifts that result in material changes in the focal length of the objective. When that happens, the test length changes and a mismatch occurs between the focal position of the test surface and the position of the reference mirror. As shown in FIG. 3, the nominal design configuration when the interferometric objective 22 is focused on the test surface S is illustrated by condition B, which is optimal, while mismatched configurations caused by thermal effects are shown as conditions A and C. Specifically, condition A is caused by a temperature change that results in an increase of the focal length of the objective, while condition C is produced by a temperature change that results in a decrease of the focal length. Under condition A, assuming, for purposes of illustration, that the distance between the splitter's surface 46 and the reference surface 44 (the reference length) remains unaffected by the change in ambient temperature, when the test beam is focused on the test surface S, the reference focal point P' is shifted beyond the reference mirror 42 and the image reflected by the mirror is out of focus. Similarly, under condition C the reference focal point P'is shifted ahead of the mirror 42 with the same result. Obviously, the interplay between the coefficients of thermal expansion of the various components in the objective 22 determines exactly what happens as a result of any departure from the design temperature for the device. Moreover, the reference length does not in fact remain constant through temperature variations, but also contributes to the mismatch illustrated above. As discussed above with respect to the test focal point P, the "infinite conjugates" imaging condition is used for simplicity of disclosure, but the description can be generalized with no loss of accuracy to the "finite conjugates" imaging condition with respect to the reference focal point P' as well.
Thus, the temperature dependence of the characteristics of conventional interferometric objectives affects the performance of the instruments under varied temperature conditions. Therefore, it would be very desirable to provide a solution to this problem.